The methods employed for carrying out these processings usually consist in comparing a two- or three-dimensional image of the surface of the tyre to be inspected with a reference image in two or three dimensions of the surface of the said tyre. The general principle of these methods consists in establishing a correspondence between the image or the surface of the tyre to be inspected and the image or the reference surface, for example by superposing them, in order to determine the moulding anomalies by analyzing the differences between the two images or the two surfaces.
In the case of the tyre, the reference image of the surface may come for example from the digital data originating from the design of the tyre or, more commonly, from the digital data used to describe and to manufacture the curing mould, the said mould itself being designed to give its definitive shape to the said tyre.
The three-dimensional image of the surface of the tyre may be obtained, in a known manner, with the aid of an acquisition system capable of determining the three-dimensional relief of the tyre surface.
Matching the reference surface and the surface of the tyre to be evaluated uses methods which must be adapted to the particular case of this type of object.
Therefore, as an example, publication U.S. Pat. No. 5,715,166 describes the transformations to be made to match a reference surface with a three-dimensional image of a given object by using transformation functions such as rotations or slidings. This method is applied with good results when it is sought to match non-deformable solid objects such as metal parts, in this instance considered to be infinitely rigid. It does not apply to the tyre situation because of the deformable nature of this product.
Publication EP 1 750 089, which relates more specifically to an application designed for the inspection of tyres, proposes to divide the surface to be inspected and the reference surface into surface portions of reduced dimensions, corresponding substantially to the surface of a marking element such as a letter or a set of letters, and then sliding the said surface portions of the reference surface and of the surface to be inspected one over the other, so as to determine the optimum match between the contours of the reliefs of the two surface portions. After having carried out this local reset, the two surface portions are compared with one another in order to determine, in the zone corresponding to the surface portion, the degree of conformity of the tyre to be inspected relative to a reference.
Although the algorithms described in this publication have the advantage of dispensing, to a certain degree, with the positional differences between the model and the real tyre to be evaluated, and with the differences in fitting and inflation from one casing to another, they are close to those described in publication U.S. Pat. No. 5,715,166 in that they still assimilate the surface portions with rigid elements.
Specifically, it is observed that the tyre coming out of the mould does not exactly match the negative image of the mould in which the moulding and curing operation has been carried out, because of the elastic nature of the materials that make it up. The tyre deforms as soon as it comes out of the curing press under the action of the thermal retractions of the materials when cooling. Moreover, when fitted and inflated, the reinforcing plies take their final position and the curve of equilibrium of the inflated tyre does not necessarily match the curvature of the curing mould.
Also, it is found to be necessary to make a very precise prior adjustment of the image of the reference surface and of the acquired image of the surface of the tyre to be inspected in order to match the two surfaces for the purpose of obtaining therefrom pertinent information on the conformity of the tyre resulting from the production.
The method described in publication WO2009077539 proposes, in order to achieve this objective, to make affine transformations of the reference surface, of which the coefficient is different from 1, so as to have it coincide with the surface to be inspected, which is the equivalent of carrying out a variable elastic deformation in a particular direction of the said surface, and thereby distinct from a simple variation of scale.
It is however observed that this method does not make it possible to make the fine adjustments necessary to the perfect superposition of the surfaces and may induce incorrect judgements during the comparison of the surface to be inspected and the reference surface.